1. Field of the Invention
This invention relates to a method of controlling a feed rate of dampening water in an offset press.
2. Description of the Related Art
In an offset press, the feed rate of dampening water, as does the feed rate of ink, has a crucial influence on printing results. It is therefore necessary for the offset press to adjust the feed rate of dampening water properly.
To execute a method of automatically detecting the quantity of dampening water and controlling the feed rate thereof, an apparatus has been proposed that, for example, measures a film thickness of water on an ink kneading roller by using an infrared sensor or the like. However, such an apparatus presents difficulties in coping with environmental changes occurring in time of printing, and the apparatus itself is extremely expensive.
In Japanese Patent No. 2831107, a tone controlling apparatus has been proposed that detects densities of a solid portion and a halftone portion of a print, performs a comparison operation on the detected densities of the solid portion and halftone portion in relation to target densities of the solid portion and halftone portion inputted beforehand based on density variation characteristics of the solid portion and halftone portion occurring with variations in the feed rates of ink and dampening water, and simultaneously controls the feed rates of ink and dampening water based on results of the comparison operation.
Generally, an offset press has far more ink rollers for feeding ink to printing plates than water rollers for feeding dampening water to the printing plates. Thus, an adjustment of dampening water is reflected on prints in a shorter time than an adjustment of ink. Rather than adjusting dampening water and ink simultaneously as described in the above Japanese patent, it is desirable to adjust the feed rate of ink while taking influences of the water adjustment into account.
In view of the above, Applicants have proposed a method of controlling the feed rate of dampening water in Japanese Unexamined Patent Publication No. 2002-355950. This method uses first and second detecting patches that show different density variations on prints, with variations in the feed rate of dampening water, whereby the feed rate of dampening water may be adjusted properly along with the feed rate of ink.
The method of controlling dampening water described in the above Japanese Publication can properly adjust the feed rate of dampening water. However, this method does not take the emulsification of ink into account.
With progress of a printing operation by an offset press, ink undergoes changes in emulsification rate. The emulsification rate of ink means a proportion of water contained in the ink and, generally, is expressed in percentages of water content. When printing is done in an ink with a large percentage of water content, the emulsification rate of the ink exerts a significant influence on printing results, such as a larger halftone area larger than when printing is done in an ink with a proper percentage of water content. It is therefore desirable in controlling the feed rate of dampening water to take the emulsification rate of ink into account.